Jet grouting is generally known as a method for ground improvement, anduses an injection rod (drill rod) having an injection nozzle arranged at a bottom end of the injection rod. In the jet grouting, a pressurized air and an improving material (self-hardening material) are injected from the injection nozzle of the injection rod in a horizontal direction. The improving material injected from the nozzle breaks up in-situ soil so that the injected material is mixed with the broken soil. Typically, the injection rod advanced to a design depth in the ground is rotated and is raised in a stepwise manner by several centimeters (specifically, the injection rod is pulled up stepwisely at certain time intervals), thereby constructing an improvement body having an approximately columnar shape (cylindrical shape) with a large diameter. An overview of steps of the jet grouting method is illustrated in FIG. 20.
<Step a>
As illustrated in FIG. 20(a), a jet grouting machine 6 is installed at a position where the improvement body (column) is to be constructed. An injection rod 7 (drill rod) is assembled to the jet grouting machine 6 while being suspended by a crane. Then, the injection rod 7 is advanced to a predetermined depth in the ground to be treated, while discharging drilling water from a bottom end of the injection rod 7 and being rotated by the jet grouting machine 6.
<Step b>
After the injection rod 7 is advanced to the predetermined depth in the ground, a rotation speed (rpm) and raising speed (s/m) of the injection rod are set appropriately. Then, injection of the improving material (grout) is started. The improving material is injected at high pressure from the injection nozzle arranged at the bottom end of the injection rod 7. In-situ soil is broken up and loosened with high kinetic energy of a jet flow of the improving material.
<Step c>
Through rotation of the injection rod 7 at the set rotation speed, the in-situ soil is broken up and loosened by the jet flow of the improving material injected at high pressure, and the improving material is forcibly mixed with the broken in-situ soil. In this way, the improvement body is partially formed at the first stage. Then, the jet grouting machine 6 is actuated so that the injection rod 7 is lifted up in a stepwise manner to a second stage, a third stage, and so on. For example, a step length (length per step) is set to 25 mm. The number of steps per meter is set to forty. As described above, the improving material is injected from the injection nozzle of the injection rod at high pressure while the injection rod is rotated at the set speed in each stage. The injection rod is lifted up in the stepwise manner in accordance with the set raising speed, thereby being capable of constructing the improvement body having a substantially columnar shape.
<Step d>
After the improvement body with a predetermined dimension is constructed in the ground to be treated, the injection rod 7 is withdrawn above the ground. Then, the inside of the injection rod 7 is washed with water.